1. Field of the Invention
The invention relates to an image forming apparatus such as a copier, a laser printer or a facsimile apparatus of the electrophotographic type.
2. Description of the Related Art
In image forming apparatuses of the electrophotographic type for electrostatically transferring a toner image electrostatically formed on the surface of a photosensitive member as an image bearing member to a recording material (e.g. paper) brought into close contact therewith, there is known one using an electrically conductive transfer roller or a corona charger as a transfer member.
In the image forming apparatus, the transfer member is urged against or brought close to the photosensitive member to thereby form a transferring portion between the photosensitive member and the transfer member, and the recording material is made to pass through the transferring portion and also, a transfer bias voltage opposite in polarity to the toner image on the photosensitive member is applied to the transfer member to thereby transfer the toner image on the photosensitive member to the surface of the recording material.
As the photosensitive member used in the above-described image forming apparatus, use is often made of an organic photoconductor (OPC photosensitive member or an amorphous silicon photoconductor (hereinafter referred to as the xe2x80x9ca-Si photosensitive memberxe2x80x9d). Of these, the a-Si photosensitive member is high in surface hardness and exhibits high sensitivity to a semiconductor laser or the like and moreover, shows little or no sign of the deterioration by the repeated use thereof and therefore, is used as an electrophotographic photosensitive member in a high-speed copier, a laser beam printer (LBP) or the like.
The above-described a-Si photosensitive member, however, is manufactured by a method of making a gas into plasma by a high frequency or a microwave and solidifying it, and accumulating it on an aluminum cylinder and forming it into film and therefore, it is difficult to uniformize the plasma and place the aluminum cylinder on the center of the plasma, and it has been difficult to make the film forming condition accurate and uniform on the entire area of the surface of the photosensitive member. Therefore, at a developing position, uneven potential of the order of 20 V has occurred in the entire area of the surface of the photosensitive member, and this has led to the problem that uneven image density occurs due to this uneven potential.
The above-mentioned uneven potential occurs because
(1) a difference in capacitance occurs from the uneven film thickness during film formation and a difference occurs to the charging capability, and
(2) due to the localized difference in the quality of film attributable to the non-uniformity or the like of the film forming state, a difference occurs to the potential decay characteristic.
The above-described potential decay after charging is very great even in a dark state when the a-Si photosensitive member is used, as compared with the OPC photosensitive member, and further the potential decay of image exposure by an optical memory is increased and therefore, in order to extinguish the optical memory by the preceding image exposure, it becomes necessary to effect pre-exposure before charging.
The optical memory will be described here. When the a-Si photosensitive member is charged and image exposure is effected, an optical carrier is produced and the potential is decayed. At this time, however, the a-Si photosensitive member has much dangling bond (unconnected bond), and this becomes a localized level and catches a part of the optical carrier to thereby reduce its running property or reduced the reconnecting probability of an optically produced carrier. Accordingly, in the image forming process, as soon as an electric field is applied to the a-Si photosensitive member during the charging at the next step, a part of the optical carrier produced by exposure is liberated from the localized level, and in an exposed portion and a non-exposed portion, a difference occurs to the surface potential of the a-Si photosensitive member, and this finally becomes an optical memory.
So, it is popular to effect uniform exposure by an exposing device before charging to thereby make the optical carrier which is latent in the interior of the a-Si photosensitive member excessive so as to be uniform on the entire surface, and erase the optical memory. At this time, it is possible to increase the quantity of light of pre-exposure emitted from a pre-exposing device or approximate the wavelength of the pre-exposure to the spectral sensitivity peak (about 680-700 nm) of the a-Si photosensitive member to thereby erase the optical memory (ghost) more effectively.
However, when as described above, an uneven film thickness and the difference in potential decay characteristic by the difference in the quality of film exist in the a-Si photosensitive member, a difference occurs to the liberation of the optical carrier from the localized level because electric fields applied to between photoconductive layers differ from each other and therefore, even if the photosensitive member can be uniformly charged at a charging position, uneven potential will occur at a developing position. Also, disadvantage will occur to the charging capability because the capacitance becomes greater in the portions of smaller film thicknesses, and when the charging capability becomes lower, the uneven charging in the above-mentioned developing portion will become more remarkable.
For the reason as set forth above, the potential decay between charging and development becomes very great and potential decay of the order of 100-200 V occurs. At this time, uneven potential of the order of 10-20 V has occurred in the entire area of the surface of the photosensitive member due to the aforementioned uneven film thickness and the difference in the potential decay characteristic.
When such uneven potential occurs, the a-Si photosensitive member having great capacitance is more greatly affected thereby and uneven image density becomes remarkable because it is small in contrast as compared with the OPC photosensitive member.
It is an object of the present invention to provide an image forming apparatus which can form good images free of uneven image density.
It is another object of the present invention to provide an image forming apparatus including a photosensitive member, exposure means for exposing the photosensitive member which has been charged, storing means for storing therein a characteristic table two-dimensionally representing the potential decay characteristic of the surface of the photosensitive member, and control means for controlling the exposure means on the basis of the characteristic table stored in the storing means.
It is still another object of the present invention to provide an image forming apparatus including a movable photosensitive member, exposure means for exposing the photosensitive member which has been charged, storing means for storing therein the potential decay characteristic of the surface of the photosensitive member with respect to a direction orthogonal to the movement direction of the photosensitive member, and control means for controlling the exposure means on the basis of the potential decay characteristic stored in the storing means.
Further objects of the present invention will become apparent from the following description.